Joshua Bassett says fame came with a dark side he never saw coming ... revealing his substance abuse spiraled so badly at one point, he believes he shouldn't have survived it.

In an appearance on the "More Life with Carl Radke" podcast, the former Disney star gets brutally honest about a chapter of his life that was happening behind the scenes while his career was taking off.

Joshua says things reached a breaking point when he was 23, admitting ... "It should have killed me, the level of substance."

The singer and actor doesn't stop there, either. Joshua goes on to detail just how consumed he was by drugs ... describing a daily routine that revolved around getting his next fix and shutting himself off from the outside world.

Joshua is remarkably candid throughout the interview ... detailing how money, freedom, and drugs created a dangerous mix.

These days, Joshua appears to be in a much healthier place ... but based on his comments here, getting there was anything but easy.